Ascorbic acid, also known as vitamin C, is a dietary factor which must be present in the human diet to prevent scurvy and which has been identified as an agent that increases resistance to infection. Ascorbic acid is used commercially, for example, as a nutrition supplement, color fixing agent, flavoring and preservative in meats and other foods, oxidant in bread doughs, abscission of citrus fruit in harvesting and reducing agent in analytical chemistry.
One current method for the manufacture of ascorbic acid utilizes a modification of the original Reichstein-Grossner synthesis (Reichstein et al., Helv. Chim. Acta, 17:311 (1934); U.S. Pat. No. 2,301,811 to Reichstein; all references cited herein are specifically incorporated by reference). In this process a glucose source is converted to ascorbic acid.
During conversion an intermediate of a diacetonide of KLG is produced.
Several two stage methods exists for the manufacture of ascorbic acid. In the first stage, glucose is converted via fermentation processes to either an isolated intermediate of KLG (Sonoyama et al., Applied and Envtl. Microbiology, 43:1064-1069 (1982); Anderson et al., Science, 230:144-149 (1985); Shinjoh et al., Applied and Envtl. Microbiology, 61:413-420 (1995)) or the intermediate of the Reichstein-Grossner synthesis, the diacetonide of KLG.
The second stage, which converts either of the intermediates to ascorbic acid, proceeds by one of two reported routes. The first route, a modification of the latter steps of the Reichstein-Grossner synthesis, requires a multitude of steps whereby the intermediate is esterified with methanol under strongly acidic conditions to produce methyl-2-keto-L-gulonate (MeKLG). The MeKLG is then reacted with base to produce a metal ascorbate salt. Finally, the metal ascorbate salt is treated with an acidulant to obtain ascorbic acid. The second route is a one-step method comprising acid-catalyzed cyclization of KLG, as originally disclosed in GB Patent No. 466548 to Reichstein) and later modified by Yamazaki (Yamazaki, J. Agri. Chem. Soc. Japan, 28:890-894 (1954), and Chem. Abs., 50:5992d) and again by Yodice (WO 87/00839). The Yodice method is commercially undesirable because it uses large amounts of gaseous hydrogen chloride, requires very expensive process equipment and produces an ascorbic acid product requiring extensive purification.
Lipases, a group of hydrolase enzymes, have been used with some success in the synthesis of esters of organic acids. In particular, lipases have been utilized in the transesterification of alcohols in which the esterifying agent is irreversible, such as when vinyl acetate is used as the esterifying agent (Thiel, Catalysis Today, 517-536 (1994)). Gutman et. al., Tetrahedron Lett., 28:3861-3864 (1987), describes a process for preparing simple 5-membered ring lactones from gamma-hydroxy methyl esters using porcine pancreatic lipase as the catalyst. However, Gutman et al., Tetrahedron Lett., 28:5367-5368 (1987), later reported that substituting delta-hydroxy methyl esters for gamma-hydroxy methyl esters and using the same catalyst produced only polymers. In EP 0 515 694 A1 to Sakashita et. al., a synthesis of esters of ascorbic acid, which are acylated on the primary hydroxyl group, comprises reacting ascorbic acid with a variety of fatty acid active esters (i.e., fatty acid vinyl esters) in a polar organic solvent in the presence of a lipase.
Thus, there exists a need in the art for methods of producing (a) ascorbic acid or metal salts thereof from KLG or esters of KLG, (b) KLG from esters of KLG and (c) esters of KLG from KLG, which have high yield and high purity with little or no by-product formation and are conducted under mild conditions. Accordingly, it is to the provision of such that the present invention is primarily directed.